The invention relates to a stretcher for a transmission belt and particularly but not exclusively for a belt for transmitting motion from the drive shaft of an internal combustion engine to auxiliary elements driven by the said engine.
Stretchers are known which comprise a fixed part suitable for being rigidly connected to a support structure integral with the engine block, a movable part hinged to the fixed part about a first axis, a pulley carried by the movable part and rotating with respect thereto about a second axis distinct from the first axis and parallel thereto, and a spring interposed between the fixed part and the movable part, which loads the movable part in the direction tending to keep the pulley in engagement with the relative belt.
Variations in belt tension due to the selective actuation of the various accessories and to wear are therefore compensated by consequent variations in the deformation of the spring and corresponding movements of the axis of rotation of the pulley.
According to a known embodiment, the fixed part comprises a pin defining the first axis, and the movable part comprises a hub rotating on the pin with the interposition of a bush, also having the function of damping by friction the oscillations of the movable part under the pulsating load transmitted by the belt to the pulley, and thus to the said movable part. The spring, a spiral one, is wound round the hub and has an internal end secured to the said hub and an external end secured to a flange of the fixed part.
The known stretchers of the type briefly described have a number of disadvantages.
Because the pulley is supported in projecting manner by the movable part, the pull of the belt causes a tilting moment on the said movable part (i.e. tending to rotate the latter about an axis transverse to its own axis of rotation). The tilting moment is balanced by the restraining reaction of the fixed part through the bush; this tends, however, to stress the bush in anomalous manner, creating non-uniform wear in it.
Obviously, the effects of the tilting moment could be reduced by increasing the dimensions of the pin and the bush whilst retaining the same configuration; this would, however, involve an undesirable increase in the size of the hub and hence also of the fixed part.